In the case of fossil-fuel-fired power plants for generating electricity, a carbon dioxide-containing flue gas is formed through the combustion of a fossil fuel. In order to prevent or decrease carbon dioxide emissions, carbon dioxide must be separated from the flue gases. In general, various methods are known for separating carbon dioxide from a gas mixture. In particular, the absorption-desorption method is usually employed for separating carbon dioxide from a flue gas after a combustion process. On an industrial scale, carbon dioxide is washed out of the flue gas using an absorbent.
The usual chemical absorbents, for example monoethanolamine (MEA), display good selectivity and a high capacity for carbon dioxide CO2. However, amine solutions as detergents also bind, irreversibly, acidic minor constituents of flue gas such as nitrogen dioxide NO2 and sulfur dioxide SO2 or sulfur trioxide SO3 in the form of sulfite and sulfate and thus increasingly impair the effectiveness of the detergent in the course of the process. Formation of sulfite and sulfate takes place, under the alkaline conditions prevailing in amine-based solvents, according to the following equations:SO2+2OH−→SO32−+H2OSO3+2OH−→SO42−+H2O
To combat the problem of accumulation of sulfite and sulfate, in the case of amine solutions there is the possibility of processing by distillation. In this, the amine solution is heated, so that the highly volatile amines evaporate and are recovered by condensation and so are separated from the high-boiling contaminants.
The notable vapor pressure of the dissolved amines can admittedly be utilized on the one hand for distillation-based purification, on the other hand, during the actual purification process, through contact with the hot flue gas, amines are discharged in small amounts with the purified flue gas into the environment, which can lead to undesirable air pollution. Moreover, distillation-based purification techniques require a high energy expenditure, and per mole of separated sulfite or sulfate, two moles of the active substance remain in the residue, which must be further processed or disposed of.
Therefore, in this respect salts of amino acids for example are particularly suitable for scrubbing CO2 flue gas, as solutions of salts of amino acids do not have a measurable vapor pressure and therefore cannot be discharged with the flue gas. However, for this reason distillation-based processing is also not possible with solutions of salts of amino acids. To prevent blocking of the salts of amino acids by the acidic flue gas constituents, therefore very expensive fine purification of flue gas (polishing) is required, in order to remove sulfur oxides SOx as completely as possible from the flue gas. These techniques are very cost-intensive with respect to capital costs and operating costs.